5mm Red LED

An LED is a diode built from a semiconductor that emits photons when electrons cross its junction. Like any diode, it only conducts in one direction and has a forward voltage drop — but that drop depends on the LED's color, since color corresponds to the semiconductor's bandgap energy (red and infrared LEDs have the lowest forward voltage, blue and white the highest).

Because an LED's resistance drops sharply once it starts conducting, it must always be paired with a current-limiting resistor — connecting one directly across a supply voltage higher than its forward voltage will destroy it almost instantly.

CircuitForge lets you set the forward voltage directly, which is the one parameter that changes between LED colors in the simulation.

Red LEDs are built from gallium arsenide phosphide (GaAsP) or aluminum gallium indium phosphide (AlGaInP), materials with relatively low bandgap energy. Because photon energy is directly tied to this bandgap, a 2.0V potential is enough to push electrons across the gap to emit red (~625nm) light. This low forward voltage makes the red LED incredibly versatile — it can easily be driven from a tiny 3V coin cell or a 3.3V logic pin with enough headroom left for a current-limiting resistor.

When driving a red LED from a 5V logic output using a standard 20mA limit, Ohm's law calculates the series resistor as R = (V_supply - V_forward) / I = (5.0 - 2.0) / 0.02 = 150Ω. In practice, choosing the next common higher value — 220Ω or 330Ω — is safer and extends the LED's life significantly without a noticeable drop in perceived brightness.

A common failure mode for beginners is driving LEDs without any resistor at all. Without resistance, the LED acts nearly like a short circuit once its 2.0V threshold is crossed. Connecting a red LED directly to a 5V supply will draw hundreds of milliamps, destroying the internal semiconductor bond wires almost instantly.